I had been searching for the solution for:
/sbin/mount.vboxsf: mounting failed with the error: No such device
... and apparently there's a difference between:
apt-get install build-essential linux-headers-`uname -r` and
apt-get install build-essential linux-header-`uname -r`
The former saved me from the error.
In case you'll encounter another error with the word "protocol", just change the folder name in the edit share sectio of the virtualbox, then try the mount command again and you're good to experience relief.